Molybdenite (MoS2) is a fairly widespread mineral but occurs in very few economic deposits of its own. Most commonly it is present as an accessory mineral in copper sulphide ores, such as those in porphyry deposits. For both types of deposit, the standard method for recovery is to grind the ore such that the minerals are liberated from the gangue and treat using froth flotation. As the molybdenite and copper minerals are both sulphides, a rougher float is used to recover both metals into a concentrated slurry from which they are then separated by selective flotation. The separation is not perfect and copper minerals remain in the molybdenite concentrate and molybdenite remains in the copper concentrate.
The copper concentrate is typically sent for smelting where the molybdenum reports to the slag and is not recovered. The molybdenum concentrate is typically roasted in air to convert the sulphide to the oxide MoO3. The roasting process also results in the formation of volatile Re2O7 which is either lost or scrubbed from the off gases. Regardless, a portion of the rhenium is lost. Further, roasting inevitably produces sulphur dioxide which is becoming an environmentally unacceptable product. The comparatively small scale of molybdenite roasting makes conversion to sulphuric acid, expensive as the acid plant is small. The MoO3 is then leached in acid, the impurities are removed and then the molybdenum is precipitated as ammonium molybdate (NH4)2MoO4 from which the metal can be produced by hydrogen reduction.
Roasting is becoming increasingly unacceptable and expensive due to the necessity to capture all of the sulphur dioxide emissions. Additionally, the loss of rhenium results in financial losses from a potentially very valuable by-product.
An alternate approach is to leach molybdenum directly from unroasted ore. A previous work, (Extractive Metallurgy of Molybdenum, C. K. Gupta, CRC Press, 1992, p. 162-171) summarises work on leaching agents for molybdenite. Ozone does not react with molybdenite under ambient temperature and pressure, only at elevated temperature and/or pressure which may require an autoclave. Acidic permanganate solution reacts but the reaction is hindered by precipitation on the surface of the molybdenite unless the solution is extremely acidic (>2N H2SO4). Sulphuric acid solution with the addition of solid manganese dioxide has been used to oxidise molybdenite, but the reaction is slow. Sodium persulphate requires the presence of a catalyst such as silver ions which increases the cost of essential reagents. Dilute nitric acid oxidises molybdenite slowly, but the reaction is limited, unless the solution is acidified with sulphuric acid to dissolve the reaction product.
Chlorine dioxide ClO2 in water works rapidly, however ClO2 is explosive and this necessitates using a low solution concentration. Low oxidant concentration will result in a low slurry density thereby requiring substantially larger leach tanks to give the required residence time. Such a leaching process will lead to very low molybdenum concentrations in solution and require a much larger, and therefore more expensive, plant than a process which runs at higher slurry density.
Acid sodium chlorate NaClO3 has a slow reaction rate unless the chlorate and acid concentrations are both high. The necessity for a low pH leads to problems with losses of oxidant as intermediates formed during the reaction include chlorine which has limited solubility.
Aerated sodium hydroxide also oxidises molybdenite but is extremely slow.
Sodium hypochlorite, a chlorine-based oxidising species, was considered in the publication to be a potentially good reagent, provided it could be recycled. As an example, an electroxidation process in which a slurry of molybdenite in brine is pumped into an electrochemical cell at 35-40° C. has been described. At the anode the chloride ions are oxidised at the anode to hypochlorite/hypochlorous acid/chlorine depending upon the operating pH (G. H. Kelsall, N. J. Welham and M. A. Diaz ‘Thermodynamics of Cl—H2O, Br—H2O, I—H2O, Au—Cl—H2O, Au—Br—H2O and Au—I—H2O systems at 298 K’, Journal of Electroanalytical Chemistry, 361(1-2), 1993, 13-24.). There is also further oxidation to chlorate ClO3− ions which do not react with molybdenite unless the solution is acidic. The chlorine bearing species then reacts with the molybdenite according to:MoS2+9NaClO+3H2O=MoO42−+9NaCl+2SO42−+6H+
Unfortunately, the stability of the hypochlorite decreases with both temperature and pH. Thus using an elevated temperature and allowing the solution to decrease in pH through the inevitable production of protons during reaction both increase the losses of oxidant increasing costs. However, these losses are avoided by maintaining the solution pH through the addition of a base (lime or hydroxide). However, this in turn increases cost and complexity of the subsequent solution chemistry by adding additional metal ions.
The outlined process is only viable on sulphide concentrates and requires heated tanks for the leaching. Further, this process relies on the feed solutions of sodium chloride being of extremely high purity, if Mg or Ca is present then precipitation of their hydroxides occurs, significantly increasing the power consumption per kilogram of product. The high level of impurities being recycled in the envisaged electrolytic process would inevitably lead to reduced power efficiency for hypochlorite generation and increasing costs for the process.
A process to selectively remove molybdenite from copper concentrates has been outlined (I. H. Warren, D. M. Mounsey, Factors influencing the selective leaching of molybdenum with sodium hypochlorite from copper/molybdenum sulphide minerals, Hydrometallurgy, 10, 1983, 343-357). A hypochlorite solution was adjusted to the required pH using hydrochloric acid, the molybdenite concentrate was then added and the pH maintained automatically by the addition of 2 M NaOH using a pH-controlled solenoid valve. It was found that a pH of 9 was optimum. To avoid the need to continuously add sodium hydroxide, carbonate ions were added to act as a pH buffer. Using this buffer was only viable as the slurry density of the solution was sufficiently low to not produce enough protons to overcome the buffer capacity. The use of such a buffer would not be viable on an industrial scale unless it operated at the same low slurry density. This would greatly enlarge the necessary plant due to the higher volumes of solution required. The final solution would also be low in molybdenum concentration and remain buffered around pH 9. The low concentration would make efficient recovery of the molybdenum more difficult than from a higher concentration solution. The buffering would require the use of further reagents to adjust pH to those where molybdenum recovery processes were optimal. The necessarily large solution volume required will also need to be recycled back to the leach stage after the molybdenum has been recovered. This would require the addition of base to regenerate the buffering effect. The overall process is not especially efficient as there is a need to regenerate the carbonate buffer.
It is unlikely that the use of a carbonate buffer is economically viable due to the problems outlined above. In an industrial process the addition of base would be more economically viable as it would be capable of handling higher slurry densities during the leaching.
Two papers from BARC (T. K. Mukherjee, P. R. Menon, P. P. Shukla and C. K. Gupta, Recovery of molybdenum metal powder from a low grade molybdenite concentrate, Chemeca 88, Australia's Bicentennial International Conference for the Process Industries, Sydney, 28-31 Aug. 1988, p. 548-554; V. S. Bhave, P. Alex, R. C. Hubli and A. K. Sun, Indirect electro-oxidation process for leaching of molybdenite concentrate by hypochlorite: a modified approach, BARC Newsletter, 297, October 2008, 139-143) outline a process operating at 30-35° C. and pH 6-8 using sodium carbonate addition to maintain this pH. During prolonged oxidation chlorate ions ClO3− ions build up consuming power and reducing the available free chloride which can be oxidised to chlorine at the anode. The latter paper used calcium carbonate to adjust pH, this resulted in the precipitation of calcium molybdate which was subsequently recovered and further processed to recover the molybdenum.
All the electro-oxidation methods maintain the pH, by way of neutralising agents, to minimise the loss of hypochlorite ions.
Throughout this specification, unless the context requires otherwise, the word “comprise”, or variations such as “comprises” or “comprising”, will be understood to imply the inclusion of a stated integer or group of integers but not the exclusion of any other integer or group of integers
The discussion of the background art is included exclusively for the purpose of providing a context for the present invention. It should be appreciated that the discussion is not an acknowledgement or admission that any of the material referred to was common general knowledge in the field relevant to the present invention in Australia or elsewhere before the priority date.